1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, and more particularly to an electrophotographic photoconductor having sensitivities in the broad wavelength range from a visible region to a near infrared region.
2. Discussion of Background
Recent development of information processing systems utilizing electrophotography is remarkable. In particular, a photoprinter which converts information to be recorded into digital signals and optically records the information is excellent in the printing quality and operational reliability. This digital recording technology is applied not only to such a photoprinter, but also to ordinary copying machines, so that a so-called digital copying machine has been developed.
A copying machine which employs the conventional analog copying technology, as well as this digital recording technology, can be provided with various information processing functions, so that it is expected that a demand for such a copying machine will be increased.
At present, semiconductor diodes (LD) and light emitting diodes (LED), which are small sized and inexpensive and have high operational reliability, are in general use as light sources for the optical printer. However, the wavelength of the light emitted from LED now in general use is 660 nm, while the wavelength range of the light emitted from LD is in a near infrared region, so that the development of an electrophotographic photoconductor having sensitivities in the range from a visible region to a near infrared region is desired.
The photosensitive wavelength range of an electrophotographic photoconductor is substantially determined by the photosensitive wavelength range of a charge generating material (CGM) employed in the electrophotographic photoconductor, so that varieties of charge generating materials such as an azo pigment, a polycyclic quinone pigment, a trigonal system selenium and a phthalocyanine pigment have been developed.
Of the above-mentioned charge generating materials, many kinds of azo pigments have been developed because of the facility of the synthesis reaction thereof, and the diversification in the chemical structures thereof.
For instance, a bisazo pigment having a diphenylpolyene skeleton as disclosed in Japanese Laid-Open Patent Applications 64-79753, 59-129857 and 62-267363, and Japanese Patent Publications 3-34503 and 4-52459; and a trisazo pigment as disclosed in Japanese Laid-Open Patent Application 57-195767 can serve as the charge generating materials which have high photosensitivities in a broad wavelength range from visible through near infrared and are capable of surely retaining the charging potential. However, the photosensitivities of the above-mentioned azo pigments are not enough to cope with the trend toward small-size, high-speed information processing apparatus.
An electrophotographic photoconductor comprising a titanyl phthalocyanine pigment subjected to a dry salt milling process is proposed in U.S. Pat. No. 5,055,368. Although such a phthalocyanine pigment has sufficiently high photosensitivity in a broad photosensitive wavelength range, the crystalline form of the pigment is readily changed depending on the kind of organic solvent used in the preparation of the photoconductor. The quality stability of the phthalocyanine pigment as the charge generating material is therefore considered to be poor. In addition, the retention stability of the charging potential of the above phthalocyanine pigment is poorer than that of the conventional azo pigment.
As previously mentioned, a charge generating material (CGM) having a high photosensitivity in a broad wavelength range from visible through near infrared has not yet been developed.